A vertical field-transfer type of CCD imager has an A register exposed to illumination (radiant energy reflected from a scene) and has B and C registers masked from illumination. Radiant energy descriptive of the scene is focused onto the A register during an image integration interval (corresponding substantially to a vertical trace period) to generate charge packets descriptive of a field of video samples. These charge packets are shifted to the B register during a field transfer interval (commonly called a "pull-down" interval) occurring within the subsequent vertical retrace period. The field of video samples thus stored in the B register is progressively shifted to the C register one line each horizontal retrace and then read out serially during the following horizontal trace period while a new field of charge packets is being generated in the A register to be read out during the next vertical trace period.
As noise levels in CCD imagers are reduced, making it feasible to use them in a broadcast-quality television camera system, it becomes desirable to obtain an indication or control signal responsive solely to the integrated dark currents. Dark currents are temperature-dependent black level currents which occur in solid-state imagers, and accumulate as charge in the signal handling portion of the imager, and which appear as signal even in the absence of light. The indication signal can be used for reducing the effects of such integrated dark currents which undesirably accompany the video signals derived from the CCD imager. For example, black level correction signals derived in response to the indication signal can be differentially combined with image-responsive signals having undesirably accompanying integrated dark current responses (commonly called "field shading") to obtain image-responsive signals substantially free of integrated dark current response. Additionally, it is known to be desirable to cool solid-state imagers to reduce the level of temperature-sensitive noise, including integrated dark currents. It has been found that reducing the temperature of the imager to about +10 degrees C. from normal operating temperatures will reduce noise attributed to dark currents fourfold. Cooling of the imager may be accomplished by a thermoelectric cooler, which is a solid-state device through which current is passed in order to pump thermal energy away from a cooled surface. Integrated dark currents can be sensed for providing an indication signal representative of imager temperature or directly indicative of dark current. In a portable television camera including a thermoelectrically cooled solid-state imager, the cooler places a power demand on the camera battery which reduces operating time. The indication signal can be used by a thermal servomechanism for controlling the current draw of the thermoelectric cooler.
Consequently, it is desirable to provide an indication signal responsive to integrated dark currents for use by the camera processing circuitry for purposes such as field shading correction and thermoelectric cooler control. Unfortunately, in a field-transfer type of imager, the masked B register is subject to partial illumination due to, for example, mask misalignment or carrier diffusion. Partial illumination of the B register generates signals which may undesirably be detected during dark current measurement and result in an erroneous dark current measurement.
It is an object of the present invention to provide an accurate indication signal of imager dark current independent of such light contamination of a masked portion of the imager.